I'm thinking of doing a casting of the whole lot as one piece, including the Webley bit seen in the pic. That way I could cast up just the finned/white bit or the whole lot depending on what people want/have already.
There is a handy layout diagram of the rib position on the barrel that I used (Ta whoever). Except it shows the front scope block replacing a section of one rib, whereas I believe it should go between two ribs. The photos of a real prop back this up.
Ribbed for your pleasure (Scope blocks not fixed in place.):